This invention relates to improvements in the extrusion of synthetic thermoplastic polymer melt, and is more particularly concerned with a new and improved method of and means for effecting redistributive mixing in an extruder.
A well-known problem in the extrusion of resinous thermoplastic materials is in the attainment of satisfactorily uniform melt and mixing of the particles or granules supplied to the extruder. In a common form, such extruders comprise a hollow barrel within which the plastic material is heated for fluxing and a helical screw mixes and advances the material toward an outlet such as an extrusion die or nozzle. Some particles, and particularly high-molecular weight particles, frequently resist melting while propelled through the heated passage within the extruder, and therefore various expedients have heretofore been employed to improve melt quality and mixing, including variously constructed mixing sections which may be of the barrier type. U.S. Pat. No. 3,486,192 is one example of such an arrangement. On the other hand, various disperision mixing devices have been proposed which comprise mechanical means for breaking up particles and randomly mixing them by agitation such as churning, shaking, tumbling, tossing and the like. U.S. Pat. Nos. 3,045,283 and 3,555,616 belong in this category. Proposal has also been made to provide an equalizing head arrangement with a fluted torpedo, and U.S. Pat. No. 3,782,692 is referred to as an example of such a structure.
However, various prior attempts at solving the problem have not been as efficient as hoped for, often have required excessive energy consumption, and in general have been expensive expedients.